1. Field of the Invention
The present invention relates to a semiconductor device having resistive elements each of which is formed as a diffusion layer on an epitaxial layer of a semiconductor substrate.
2. Description of the Related Art
Semiconductor devices, such as bipolar integrated circuits, include resistive elements each of which is formed as a diffusion layer on an epitaxial layer of a semiconductor substrate. In the diffusion layer, doping elements are diffused into the semiconductor substrate.
FIG. 7A and FIG. 7B show a resistive element in a conventional semiconductor device. FIG. 7A is a cross-sectional view of the resistive element, and FIG. 7B is a top view of the resistive element.
As shown in FIG. 7A and FIG. 7B, in the conventional semiconductor device 1, an n-type epitaxial layer 4 is formed on a p-type silicon substrate 2 such that the epitaxial layer 4 is isolated by a (p+)-type isolation layer 3. By using a commonly known impurity doping process, various diffusion layers are formed on the epitaxial layer 4 to provide various circuit elements, such as resistors, transistors or diodes, on the silicon chip.
In the conventional semiconductor device 1, as shown in FIG. 7A and FIG. 7B, a p-type base diffusion layer 5 is provided on the n-type epitaxial layer 4 so that the resistive element is formed as the diffusion layer 5 on the epitaxial layer 4. At this location, a p-n junction between the diffusion layer 5 and the epitaxial layer 4 is formed, and the resistive element acts as a resistor having a resistance of the diffusion layer 5.
When the p-n junction between the diffusion layer 5 and the epitaxial layer 4 is connected to a forward-bias current source (not shown), the forward bias results in the movement of majority charge carriers (or electrons) into the opposing halves of the p-n junction. In this case, the electric potential of the diffusion layer 5 is higher than the electric potential of the epitaxial layer 4. A leaking current flows across the p-n junction.
In the resistive element of the above type, a bias voltage xe2x80x9cVbiasxe2x80x9d is initially applied to the epitaxial layer 4 of the p-n junction such that the electric potential of the epitaxial layer 4 is initially higher than the electric potential of the diffusion layer 5. As shown in FIG. 7A, an additional (n+)-type diffusion layer 6 is formed on the epitaxial layer 4, and one of a plurality of electrodes 8 is connected to the diffusion layer 6. The bias voltage Vbias is supplied to the epitaxial layer 4 of the p-n junction through the electrode 8 and the diffusion layer 6.
In the conventional semiconductor device 1, as shown in FIG. 7A, a dielectric layer 7 of silicon dioxide (SiO2) and the electrodes 8 of aluminum (Al) are provided on the silicon substrate 2, and a protective layer 9 of silicon nitride (SiN2) is formed thereon to cover the dielectric layer 7 and the electrodes 8.
In the resistive element of the conventional semiconductor device 1, the bias voltage Vbias, which is initially applied to the epitaxial layer 4, results in the reverse bias between the diffusion layer 5 and the epitaxial layer 4. The reverse bias may create the movement of the charge carriers away from the interface in the p-n junction, and an insulating zone forms. In such a case, the voltage dependence of the resistive element on the base diffusion layer 5 will be developed due to the insulating zone in the p-n junction. Namely, the resistance of the resistive element considerably varies when the voltage supplied to the epitaxial layer 4 of the resistive element is slightly changed.
The voltage dependence of the resistive element will be detrimental to the operational characteristics of the semiconductor device, and such is true of a case in which the resistive element is formed as a feedback resistor in an inverting amplifier device.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a semiconductor device in which resistive elements are connected to effectively prevent the variation of the operational characteristics of the semiconductor device regardless of whether the applied voltage between contacts of each resistive element varies.
According to one preferred embodiment of the present invention, a semiconductor device having a plurality of resistors each having a resistance that varies depending on voltage applied between contacts of the resistor, comprises: a first resistor which includes a first resistive layer connected at a first contact to a first input wire and connected at a second contact to a first output wire, the first output wire having a first shielding portion which is connected to the second contact and shields the first resistive layer; and a second resistor which includes a second resistive layer connected at a third contact to a second input wire and connected at a fourth contact to a second output wire, the second output wire having a second shielding portion which is connected to the fourth contact and shields the second resistive layer, wherein the first resistor and the second resistor are connected such that a potential difference of the first shielding portion of the first resistor from the first contact thereof and a potential difference of the second shielding portion of the second resistor from the third contact thereof are equal in polarity.
In the semiconductor device of the above preferred embodiment, the first resistor and the second resistor are connected such that the polarity of the first shielding portion of the first resistor at the second contact matches with the polarity of the second shielding portion of the second resistor at the fourth contact. For example, when the polarity of the applied voltage between the contacts of the first resistor is positive, the polarity of the applied voltage between the contacts of the second resistor is also positive. The semiconductor device of the above preferred embodiment includes the first and second resistors which are connected as described above, and it is possible to keep the resistance ratio of the first and second resistors constant regardless of whether the applied voltage varies.
Therefore, the semiconductor device of the present invention is effective in preventing the variation of the operational characteristics of the semiconductor device regardless of whether the applied voltage varies. In a case in which the semiconductor device is formed as an amplifier device including an operational amplifier, it is possible to effectively prevent the variation of the amplification degree of the amplifier device regardless of whether the applied voltage between the contacts of each resistor varies. It is possible to improve the linearity and the distortion characteristics of the amplifier device.